Ooh, you can help me.
What are the best certs for engineer for anti-vehicle, and what should I get on my flash? (I'm thinking the boost or proximity radar).
A fully dedicated AV engineer would mean;
AV turret (1000 certs or 700 SC)
AT mines (100/200 certs)
C4 (200/500 certs)
Flak armour (1/10/50/150/1000 certs)
Sticky grenades (100 certs)
S type carbine plus UBGL (500 cert or 700SC weapon, 100 cert UBGL, another 230 for other attachments and scope)
Utility pouch rank 4 for 2 each extra C4/tank mines (200/400/500/1000, C4/tank mines alternate ranks)
Sticky grenades are there to more reliably detonate mines when planted under a stationary Sunderer which may not always work into the future. They are also useful in AI situations (stick one to an enemy and watch him run into all his friends trying to escape) so not a waste of certs. UBGL fills the same role as well as doing damage to finish off burning vehicles, but can be buggy and tricky to use at times. Flak is obvious in my opinion.
As a priority though? 100 on tank mines. Even with the nerf they are incredibly useful. 200 on C4 and 200 utility pouch is the cheapest way any class can carry 2 bricks of C4 so is a reasonable option. As Metalax said, it gives you AV options while your mines are planted.
In the end the AV turret is pretty damned powerful, so may be worth a SC purchase. If not then prioritise that over most of the other expensive stuff.
For myself I prefer running engineer as a support and defensive class so went for AI mines over C4 to help in indoor defences. I bought my C4 on the heavy assault. I can still use tank mines directly against vehicles in suicide charges, but for AV when my tank mines are laid I pull a HA. The rocket launcher works better in combination with C4 than any engineer option short of being able to carry 3 or 4 bricks with the utility pouch.
As for the Flash I can only really speak to my own build. Bold is where I've stopped so far;
Scout radar (50/100/200/
500 certs for 25/50/75/
100m radius)
Stealth (
30/50/100/150/200/400/500/1000 certs)
M12 Kobalt (
100 cert or 250 SC unlock) plus;
Zoom optics (1/
50/150/500 certs for 1.25x/
1.5x/1.75x/2x)
Maximum ammo (1/
10/30/50/100/150/200/400/500/1000 certs, 100 extra rounds per rank)
The Kobalt gives me a more accurate version of the engineers turret on a mobile platform. Rank 1 stealth means you don't appear on people's radars unless you are spotted while the scout radar will help you spot anyone around. Higher ranks just reduce the time taken for lock-on weapons to lock-on, but that happens so rarely it's not worthwhile. The 100m radar is expensive but gives you full radar vision over nearly any base in the game, including biolabs when positioned underneath. I use this as my primary transport, riding as a heavy assault. This gives me the most AI/AV options while also acting as a scout radar for any allies I'm moving along with.
As a side note, the zoom optics don't replace each other when you cert up but rather act as different scopes, so you can choose from any of the zoom levels you have previously unlocked. So if you don't like a higher zoom level you can downgrade.
I'm probably going to buy the scrapper chassis with this to help with the mobility inside bases. Getting caught up on obstacles or unable to turn to target enemy infantry gets me killed more than a lack of speed does.
If you wanted to go all out AV then you would want either wrath cloaking or the turbo to get into and out of range and the Fury grenade launcher to deal actual damage to enemy armour. I'd stick with stealth to hope to get some shots off before I'm noticed rather than pray that composite armour lets me survive. Definitely racer in that setup.